Generally, the present invention relates to thin film formation on substrates. In particular, the present invention relates to forming a highly uniform and continous thin film on a substrate using a resealable vial carrier containing thin film forming amphiphilic material.
Hydrolyzable alkyl silanes and polymeric amphiphilic molecules having intrinsic ability to self-assemble in a thin film are used to form thin films on various substrates such as glass, metals, plastics and anti-reflective surfaces. These thin films have been used to render different properties on the substrate surface, such as easy clean, water repellent, non-stick, chemical and corrosion resistance properties. Forming a durable thin film of amphiphilic molecules directly on the plastic surface, such as plastic lens, is difficult. So a metal oxide such as silicon dioxide “silica” layer is initially applied on plastic lenses, inside a high vacuum chamber under anhydrous conditions and then the thin film of amphiphilic material is applied.
In general, most of the plastic lenses coated with anti-reflective (AR) film have a last layer as silica. The silica coated lens is then exposed to water vapors in the clean ambient air to hydrolyze the silica surface, converting —SiO2 to —Si—OH bonds. The —Si—OH bonds on the silica surface enhance strong adhesion between amphiphilic molecules and the lens surface. The silica coated hydrolyzed lenses are then put in a second low vacuum chamber for deposition of thin film of hydrolyzable amphiphilic molecules. Coating the thin film of hydrolyzable amphiphilic molecules in the first high vacuum chamber creates contamination and corrosion inside the chamber. Thus, the vacuum chamber cannot be used again for AR film coating until it is thoroughly cleaned.
Hydrolyzable amphiphilic molecules such as chlorosilanes have been used in solution to form thin films on substrates. By way of example, description of such materials and their ability to form thin films are contained in: W. C. Bigelow et al, J. Colloid. Sci., 1, 513-538(1946); L. H. Lee, J. Colloid. & interface Sci., 27,751-760 (1968); E. E. Polymeropoulos et al. J. Chem. Phys., 69, 1836-1847, (1978); and U.S. Pat. Nos. 4,539,061; 5,078,791; 5,106,561; 5,166,000; 5,173,365; 5,204,126; 5,219,654 and 5,300,561, the disclosures of which are hereby incorporated herein by reference.
Typically, the hydrolyzable amphiphilic material is dissolved in a solvent and the substrate to be coated is brought in contact with the solution for a period of time. After adequate exposure, the substrate is cleaned with water and soap solution. Forming the thin film in this manner has certain problems. For example, the solvent is toxic and may also evaporate into the air creating a flammable hazard. Also, the amphiphilic molecules in solution may hydrolyze due to exposure to air borne moisture, which creates hydrochloric acid as a by-product. This poses additional health hazards to the operator. Also, the used old solution of amphiphilic molecules needs to be disposed off according to local and federal regulations. Thus, there can be substantial environmental, health and waste disposal issues with this prior art method.
Another method to form a thin film of amphiphilic molecules on a substrate involves using a rupturable glass ampoule to deliver the liquid amphiphilic material in a vacuum chamber and coat the substrate surface. This method has been described in U.S. Pat. Nos. 6,171,652 and 6,206,191, which are hereby incorporated by reference. According to the method, a vapor phase coating process is used. A sealed glass ampoule containing a thin film forming liquid amphiphilic material is placed inside a low vacuum chamber together with the substrate to be coated. According to this process, as vacuum is established inside the chamber, the glass ampoule is broken open via a mechanical device in the chamber and the amphiphilic material is vaporized by heating. The vaporized amphiphilic material then proceeds to form a thin film on the substrate. The coated lenses are then removed from the chamber and the empty broken glass ampoule is discarded.
Although the sealed frangible ampoule serves as a good carrier to charge the vacuum chamber with a liquid thin film forming material, several problems still exist with its use. First, when the frangible ampoule is broken open, glass fragments can scatter throughout the chamber potentially impinging upon or otherwise causing damage to the substrate. This is caused by the pressure difference between the inside of the ampoule and the vacuum chamber. Second, when the ampoule breaks open with high force as previously described, the contents of the ampoule tend to suddenly spurt out. This leads to a non-uniform film on the substrate including spots and drops of film forming material on the substrate. This requires the coated substrate to be inspected and often cleaned as a result. Third, since a mechanical device inside the chamber is used to break open the ampoule, corrosion and mechanical failure of the mechanical device can cause the ampoule not to break open properly. This can result in the film forming material not being released in the chamber, and no coating on the substrate. This results in lost time and decreased production. Accordingly, such ampoule carriers are not reliable dispensing devices. Also, since the process uses a low vacuum chamber, solid thin film forming polymeric amphiphilic materials cannot be used in the sealed ampoule that requires very high vacuum to vaporize. As such, the sealed ampoule is not a versatile carrier.
Another recent development to deliver solid polymeric amphiphilic materials, such as the polyhedral oligomeric silsesquioxanes (POSS), is described in U.S. Pat. No. 6,881,445 and is hereby incorporated herein by reference. A porous metal carrier (e.g. a tablet) is impregnated with the thin film forming solid material and placed in side the high vacuum chamber together with the substrate to be coated. After the desired level of vacuum is achieved, the porous carrier is heated at high temperature to evaporate the solid amphiphilic material in the chamber thereby coating the substrate.
As described, the porous carrier may work but it still has certain limitations and disadvantages. First, the porous carrier is not suitable for thin film forming moisture sensitive amphiphilic molecules such as hydrolyzable chlorosilane liquids. As the porous carrier is exposed to air during use, the amphiphilic molecules hydrolyze, thereby giving off toxic by-product and poor quality film on the substrate. Also, holding a liquid in the pores of a porous carrier is difficult or impractical since the liquid tends to leak or flow out of the pores of the porous carrier. As such, only semi-solid or solid amphiphilic materials can be used to impregnate the porous carrier. Second, impregnating solid polymeric amphiphilic materials into porous carrier requires solvents that are highly volatile and toxic to the worker. Preparing the porous composite also requires heating at high temperature and or vacuum distillation. The porous carrier composite, as defined, does not have the versatility to be used reliably with liquid amphiphilic materials in a low vacuum chamber.
It is therefore an objective of the present invention to provide an improved thin film coating method using a more reliable, versatile and safer dispensing system (material carrier) containing thin film forming amphiphilic materials.